With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure and LiMnO2, LiCoO2, LiCo1-xNixO2 and LiNiO2 having a rock-salt type structure, or the like. Among these positive electrode active substances, LiCoO2 is more excellent because of a high voltage and a high capacity thereof, but has the problems such as a high production cost due to a less amount of a cobalt raw material supplied, and a poor environmental safety upon disposal of batteries obtained therefrom. In consequence, there have now been made earnest studies on lithium manganate particles with a spinel type structure (basic composition: LiMn2O4; hereinafter defined in the same way) which are produced by using, as a raw material, manganese having a large supply amount, a low cost and a good environmental compatibility.
As is known in the art, the lithium manganate particles having a spinel structure may be obtained by mixing a manganese compound and a lithium compound at a predetermined ratio and then calcining the resulting mixture at a temperature of 700 to 1000° C.
When using the lithium manganate particles as a positive electrode active substance for lithium ion secondary batteries, there tends to arise such a problem that the resulting battery has a high voltage and a high energy density, but tends to be deteriorated in charge/discharge cycle characteristics. The reason therefor is considered to be that when charge/discharge cycles are repeated, the crystal lattice is expanded and contracted owing to desorption and insertion behavior of lithium ions in the crystal structure to cause change in volume of the crystal, which results in occurrence of breakage of the crystal lattice or dissolution of manganese in an electrolyte solution.
At present, in the lithium ion secondary batteries using lithium manganate particles, it has been strongly required to suppress deterioration in charge/discharge capacity due to repeated charge/discharge cycles, and improve the charge/discharge cycle characteristics, in particular, under high-temperature and low-temperature conditions.
In order to improve the charge/discharge cycle characteristics of the batteries, it is required that the positive electrode active substance used therein which comprises the lithium manganate particles has an excellent packing property and an appropriate size, and further is free from elution of manganese therefrom. To meet the requirements, there have been proposed the method of suitably controlling a particle size and a particle size distribution of the lithium manganate particles; the method of obtaining the lithium manganate particles having a high crystallinity by controlling a calcination temperature thereof; the method of adding different kinds of elements to the lithium manganate particles to strengthen a bonding force of the crystals; the method of subjecting the lithium manganate particles to surface treatment or adding additives thereto to suppress elution of manganese therefrom; or the like.
Conventionally, it is known that aluminum is incorporated in the lithium manganate particles (Patent literature 1). In addition, it is known that a sintering aid such as boron oxide, boric acid, lithium borate and ammonium borate is added upon production of lithium manganate to attain effects by addition of the sintering aid (Patent literature 2). Further, it is known that a content of sulfur in lithium manganate is reduced (Patent literature 3).
In addition, various attempts have been made to improve properties of lithium manganate by using a Zr oxide as an additive or a coating agent. For example, there is described a method in which Li2ZrO3 is formed on a surface layer of respective lithium manganate particles like a core/shell structure to improve properties thereof (Patent Literature 4). Also, it is described that a sheet-like material comprising Li2ZrO3 is incorporated as a gas adsorbent into a battery to suppress generation of gases therein (Patent Literature 5).